In material transfer systems such as fluid transfer systems, it is often desired to assess the level of material in a vessel in order to determine when to initiate a control event. These control events could include turning on or off a pump, opening valves or drains, or adding material to the vessel. In the field of fluid transfer, as the fluid level in a reservoir becomes too high, the fluid is often transferred from the reservoir and discharged at another location such as another reservoir or into the environment. In sump pump applications, a basin is used to collect wastewater. When the level of wastewater rises to a pre-determined high point, the pump is switched on to drain the basin.
A number of devices, including ultrasonic sensors, capacitive sensors, thermal sensors, and float switches, are used to determine the water level in the vessel and to initiate a control event based on the water level. A float switch has moving parts which are prone to hanging up on solids in the wastewater or on other parts in the pump system, often causing the pump to malfunction.
One method to eliminate issues with moving parts in a pump system is to sense the overall electrical current that a pump uses to determine when the pump begins to take on air instead of wastewater, which would indicate a low water level in the basin. In a typical sump pump application, the pump draws a normal running current while pumping wastewater. When a large amount of air gets into the pump, the running current drops to a lower level. However, the same current drop can occur in any situation where the pump is pumping against a total head pressure and that pressure goes down. For example, a current drop may occur when a solid is caught in the discharge line or when the discharge line is being moved to a point of higher elevation. Because a number of events can affect the current drawn by the pump, relying on a current drop to detect a low fluid level in the basin may lead to a misdiagnosis of the fluid level and, consequently, an unreliable pump system.